1. Field of the Invention
The present invention relates to the use of a steel for the manufacture of structural tubes of structural elements which are subjected to strong mechanical loads and stresses, particularly for door reinforcements in the manufacture of automobiles.
2. Description of the Related Art
In order to provide protection against side impacts during automobile accidents, the vehicle doors are frequently stiffened by means of reinforcing elements which partially absorb the kinetic energy of the impacting vehicle and convert the kinetic energy into plastic deformation. In order to be able to be used for this purpose, the steel tubes must meet relatively high requirements with respect to strength, ductility and working capacity. The same is true, for example, for structural tubes for manufacturing stabilizer or other structural components which are subjected to strong mechanical loads or stresses.
Structural tubes of this type are usually hot rolled, wherein the final rolling temperature is between 900.degree. C. and 1080.degree. C. The necessary strength properties can be adjusted in dependence on the used type of steel by water hardening. For example, a known steel which is manufactured in accordance with this method contains 0.18% C, 0.4% Si and 1.14% Mn, the remainder being iron and usual impurities. The significant disadvantage of this method is the fact that it requires additional heat treatment of the steel tube for adjusting the mechanical properties. On the one hand, an additional heat treatment makes the manufacture of such tubes more expensive. On the other hand, when hot forming operations are performed on the tube during the further processing thereof, for example, into door reinforcement tubes, the mechanical properties of the tube are changed in such a way that the strength properties may unintentionally be significantly lowered as compared to the initial condition of the tube. The same may occur in the heat influence zone of welding seams which may be necessary for assembly.
DE 37 28 476 C1 and DE 39 35 965 C1 disclose two other steel materials for reinforcement tubes which obtain their strength properties solely by an air cooling from the rolling heat as a result of their chemical compositions, so that no separate heat treatments are required. However, the use of these steels results in other significant disadvantages. Thus, both steels cannot be manufactured in a single-stage smelting process on a large scale by means of the LD method. This is because the high proportion of alloying elements makes it necessary to adjust the chemical composition in two partial stages which inevitably leads to increase in the expenses of producing the initial material. The two steels have the following chemical compositions in percent by weight:
______________________________________ DE 37 28 476 C1 DE 39 35 965 C1 ______________________________________ max. 0.35% C 0.15-0.25% C max. 0.50% Si max. 0.60% Si max. 1.80% Mn 3.4-6.1% Mn max. 0.030% P max. 0.030% P max. 0.030% S max. 0.030% S 0-1.5% N 0-1.0% Ni 1.8-2.2% Cr 0-1.0% Cr 0.4-0.7% Mo 0-1.0% Mo 0.025-0.050% Al max. 0.005% Al 0-0.15% V ______________________________________
the remainder being iron and usual impurities.
In order to adjust the required mechanical properties of the steel according to DE 37 28 476 C1, it is necessary to add large quantities of the elements Cr, Ni and Mo. However, an alloying concept on the basis of these elements is a relatively expensive solution mostly because of the costs of the alloying elements.
The steel described in DE 39 35 965 C1 has a high resistance to tempering which favorably influences the properties of the steel. These properties prevent a significant reduction of the strength properties during subsequent hot forming or during hot galvanizing. On the other hand, a disadvantage is the fact that the cold forming capability of this steel is extremely limited, as is the case in the steel of DE 37 28 476 C1. As a result, certain tube dimensions, as they can be manufactured directly by means of conventional hot forming, cannot be produced from this steel. DE 40 32 996 Al discloses another steel for the manufacture of steel sections which are to be used as door reinforcements and have an outer corrosion protection by galvanizing. The steel has the following composition:
0.18-0.25% C PA1 0.30-0.50% Si PA1 1.30-2.00% Mn PA1 0.1-0.5% Cr PA1 0.1-0.3% Mo PA1 0.02-0.07% Ti PA1 0.002-0.007% B PA1 0. 15-0.30% C PA1 0.50-0.80% Si PA1 2.05-3.35% Mn PA1 max. 0.03% P PA1 max. 0.03% S PA1 0.50-1.00% Cr PA1 max. 0.60% Mo PA1 max. 0.05% Al PA1 0.01-0.05% Ti PA1 0.0015-0.0035% B PA1 0.002-0.015% N PA1 single-stage steel production by means of the LD process; PA1 inexpensive alloying elements; PA1 air hardening capability; PA1 high-resistance to tempering; PA1 high strength properties; and PA1 high energy absorption capacity.
the remainder being iron and usual impurities.
Accordingly, this steel is a water hardening steel whose mechanical properties can only be adjusted after hot rolling by carrying out a special and separate heat treatment. Although the steel has the characteristics of a water hardening steel, steel tubes manufactured from this steel can be protected against corrosion by galvanizing them, without unduly reducing the adjusted strength properties by the resulting heating of the tubes.